I. Field of the Invention
The present invention relates generally to crankshafts and, more particularly, to a method and apparatus for constructing a multi-piece crankshaft.
II. Description of the Prior Art
In a conventional crankshaft, the crankshaft includes a segmented main shaft having an axis aligned with the axis of rotation of the crankshaft. One or more crankpins are also provided at a position radially spaced from, but parallel to, the main shaft. The piston is then secured to each crankpin which rotatably drives the crankshaft about its axis of rotation.
Since the crankpins are radially offset from the axis of rotation of the crankshaft and attached to a piston, counterweights are conventionally provided for securing the main shaft to the crankpins so that the overall crankshaft has a balanced rotation. In many previously known crankshafts, the entire crankshaft is made from a single heavy body of cast metal which is then machined so that the main shaft, crankpins and counterweights are of a one-piece construction. Machining such crankshafts, however, is necessarily expensive which increases the overall cost of the crankshaft.
However, there have been crankshafts which are constructed from multiple pieces for low cost construction. Such crankshafts are oftentimes used in small two cycle engines although they can be used in other types of engines or compressors.
Typically, the counterweights are constructed from an inexpensive material, such as powdered metal or steel stampings, and the counterweight has holes formed through it corresponding to the position of the crankpin and main shaft. The crankpin and main shaft are then constructed from conventional round stock. The ends of the main shaft and crankpins are knurled, splined or otherwise deformed and pressed into the openings formed in the counterweight to thereby form the crankshaft.
One primary disadvantage of these previously known multi-piece crankshafts is that the main shaft and crankpins must be parallel to each other within very high tolerances. However, during the pressing operation, the main shaft and crankpins often become skewed relative to each other and the resulting crankshaft assembly must be either corrected by bending the crankpin and main shaft relative to each other or, in some cases, discarded as scrap. Furthermore, these prior art crankshafts were also subject to failure from push out and twisting of the shaft relative to the counterweight.
The present invention provides a multi-piece crankshaft assembly and method for making the same which overcomes all of the above-mentioned disadvantages of the previously known multi-piece crankshaft constructions.
In brief, the crankshaft assembly of the present invention comprises a crankpin and a main shaft, both of which are cylindrical in cross-sectional shape. One end of both of the main shaft and crankpin includes an indentation, such as spline, knurling or the like.
The crankpin and main shaft are then placed in cylindrical positioning recesses in a mold having a mold cavity corresponding to the desired shape of the counterweight. With the crankpin and main shaft positioned within the mold positioning recesses, the ends of the crankpin and main shaft which includes the indentation are in registration with the mold cavity corresponding to the counterweight. Furthermore, the positioning recesses in the mold are machined with high precision which ensures parallelism between the main shaft and crankpin within the mold.
With the crankpin and main shaft positioned within the mold as described above, the mold cavity corresponding to the counterweight is then filled by pouring, injection or the like with a liquid thermosetting material, such as zinc. This liquid material not only fills the mold cavity but also the indentations in both the crankpin and main shaft. Consequently, upon hardening of the material, the material fills the indentations in the crankpin and main shaft, thus precluding either rotational or longitudinal movement between the counterweight and both the crankpin and main shaft.
Since the main shaft and crankpin are maintained parallel to each other within very small tolerances, once the liquid material has set, parallelism between the crankpin and main shaft is established for the completed crankshaft assembly.
In a modification of the present invention, the formed crankshaft includes two spaced counterweights with a crankpin extending between the counterweights. The crankpin, however, includes a throughbore. Furthermore, with the crankpin positioned in the mold, one end of the crankpin is positioned within a mold cavity corresponding to one counterweight while the opposite end of the crankpin is positioned within a second mold cavity corresponding in shape to a second counterweight.
Consequently, when the liquid metal is cast into the mold, the liquid metal not only fills the cavities corresponding in shape to the counterweights, but this liquid metal flow also flows inwardly from each end of the crankpin. The metal flow from the ends of the crankpin meet and flow together in substantially the center of the crankpin so that, upon cooling of the liquid metal, the cooled liquid metal forms a continuous connection from the first counterweight, through the interior of the crankpin and to the second counterweight thus rigidifying the connection between the counterweights through the crankpin.